Gavin Henson is a doubt for the opening match in Wales's defence of their Six Nations title against Scotland at Murrayfield on Sunday after straining a calf muscle in training this week. The centre, who was 27 last Sunday, has not played for Wales since their grand slam clinching international against France last March and a decision on his fitness will be made before the squad flies north.

The Wales coach, Warren Gatland, has paired Henson with Jamie Roberts in midfield for the second time. The first occasion was back in November before the opening match of the autumn international series against South Africa, only for Henson to pull out less than 24 hours before kick-off with an achilles tendon problem having missed the summer tour to South Africa following ankle surgery.

He was replaced then by Tom Shanklin, with Roberts moving to inside-centre and Andrew Bishop coming on to the bench. The same will happen tomorrow should Henson pull out which, given his reluctance to play unless he is 100% fit, is more likely than not.

"Gavin felt the problem in training on Tuesday and he is having treatment," said Gatland. "We will not wait until the last moment before making a decision about him, as we did in November, and we will make the call on Friday. The prospect of Jamie and Gavin pairing up in the midfield is an exciting one, from both an attacking and defensive point of view. They are both big men and looked sharp together in training last week."

Gatland has chosen the pack that started in the victory over Australia last December, meaning the fit-again hooker Huw Bennett makes only the bench, but the three changes behind see the return of Henson and the scrum-half, Mike Phillips, with Leigh Halfpenny who, like the No8, Andy Powell, will be making his championship debut, coming in on the wing for Mark Jones, who has only just recovered from an appendix operation.

Wales are the favourites to retain the title but it is 100 years since they won back-to-back grand slams. "We are not huge favourites," said Gatland. "We played well in the autumn, producing some positive rugby, but we have some improving to do. It used to be the case that every Six Nations people talked about England or France winning the title. At least that has changed, but any one of five sides can win the championship."

It is a year since Gatland took charge of Wales for the first time. The transformation from a side that had failed to reach the World Cup quarter-finals little more than three months earlier to one that swept all before them in the championship before becoming the only Six Nations side to record a victory over one of the southern hemisphere big three in 2008 has been marked but it is more than 30 years since Wales last managed to sustain success.

"Successful coaching is about creating an honest environment in which everyone is heard," said Gatland. "When we took over, we weeded out some rubbish in the squad and we have a group of players who get on well.

"We want to be challenged as coaches and have players question and disagree with us. That is how we improve as a management.

"I have been with other teams in the past and seen how players respond to success. In some cases egos grow and preparation suffers but that is not the case with this group of players. Our training session last Saturday was as tough and ruthless as any I have conducted. A year ago the players would not have been able to cope but they have grown."

Like Henson, Phillips returns to the side for the first time since last season's Six Nations after coming through two operations on his knee. "I went through some low points during the months I was out of action but Warren kept in touch and that meant a lot to me," said the 26-year-old scrum-half.

"There is a lot of competition in my position with Dwayne Peel not making the original squad but it is the same throughout the side. I have got hold of the jersey again and do not intend to let it go."

Murrayfield has not been the happiest of venues for Wales; they have won there only twice since 1985. "We always seem to have nightmares there," said the flanker, Martyn Williams. "It is a really tough place to go and our recent record there guarantees that no one will be taking anything for granted on Sunday."